In a mobile communication system, transmit diversity may be used by providing multiple transmit antennas to a communication apparatus for improved transmission quality. There are two types of transmit diversity, open-loop type transmit diversity and closed-loop type transmit diversity. In the open-loop type of transmit diversity, the transmit antennas are selectively used in accordance with a predefined selection pattern. In the closed-loop type of transmit diversity, an appropriate one of the transmit antennas is selected in accordance with feedback from communication opponents.
FIG. 1 illustrates an exemplary application of the closed-loop type of transmit diversity to uplinks. First, it is determined whether respective transmit antennas have good channel states based on reference signals transmitted from the respective transmit antennas. The quality of the channel state may be represented in a channel quality indicator (CQI), for example. Then, a transmit antenna associated with a better channel state is determined, and the determination is fed back to the transmitter as antenna selection information.
FIG. 2 illustrates an exemplary transmission of reference signals and data signals from two different transmit antennas in the case where a user apparatus includes the transmit antennas. The data signals are transmitted from a transmit antenna with a better channel state. As a rule, the reference signals are transmitted from the same transmit antenna as the data signals, but at a certain cycle, the reference signals are transmitted from a transmit antenna different from the data signals. For example, the reference signals may be transmitted via the same transmit antenna as the data signals in the first through third subframes of four subframes while the reference signals may be transmitted via a transmit antenna different from the data signals in the fourth subframe. This makes it possible to select an appropriate one of the transmit antennas under reliable comprehension of transmission conditions on the respective transmit antennas.
For example, the closed-type ASTD for uplink transmissions is disclosed in 3GPP, TSG RAN WG1 Meeting#47bis, R1-070097, January 2007.